1. Technical Field
The present invention relates to a technique of driving scan lines using a multiplexer.
2. Related Art
As for electro-optical devices, such as liquid crystal devices (LCDs), pixels are disposed at every intersection of a plurality of scan lines and a plurality of data lines. Each of the pixels becomes the gray-scale according to the voltage (or current) of the corresponding data line when the corresponding scan line is set to an active level (for example, H level), and maintains the gray-scale even after the corresponding scan line becomes a non-active level (it is L level if the active level is an H level). Therefore, it is possible to display a target picture by activating a plurality of scan lines to an active level in predetermined turn and supplying a voltage (or current) corresponding to the gray-scale to the pixels associated with the activated scan lines having an active level via the data lines.
Here, a circuit which activates the plurality of scan lines in the predetermined turn so that the scan lines have an active level is called a scan line driving circuit and generally a shift register is used as the scan line driving circuit. As for such a scan line driving circuit, it is preferable that the scan line driving circuit is made into a peripheral circuit built-in type in which the scan line driving circuit is implemented with a switching element such as the pixel rather than the scan line driving circuit is implemented by mounting an external integrated circuit from the viewpoint of improvement in the manufacturing efficiency which can be resulted from harmonization of a process.
By the way, although a shift register has a complementary type logical circuit (an inverter or a clocked inverter) which is the combination of a p-channel type transistor and an n-channel type transistor, if electrical properties of the p-channel type transistor and the n-channel type transistor are not harmonized, there is a trouble in that penetration current flows. For such a reason, JP-A-2002-169518 discloses a so-called demultiplexer system in which a plurality of scan lines is grouped into a plurality of blocks, each including a predetermined number of rows of scan lines (for example, three lines), transistors TFT are provided to the scan lines, respectively as a switch, the blocks are selected one by one by address signals, the switches of the plurality of scan lines in one selected block are turned on one by one in turn by a select signal, and the scan lines are activated in turn to have an active level.
However, the known technique disclosed in JP-A-2002-169518 has a problem in that a voltage higher than the active level of the scan lines by a threshold voltage or more of the transistor serving as the switch must be applied to a gate electrode of the transistor in order to turn on the transistors disposed at the scan lines. Accordingly, with the technique, it is necessary to generate a voltage higher than the active level separately, which leads to the increase of a breakdown voltage of a power supply circuit which generates such voltage, and the increase in complexity of the structure.